Voltage source inverters are commonly used for controlling electrical loads such as motors or generators with controllable frequency. Voltage source inverters have a direct current intermediate circuit, through which energy is transferred as a DC voltage to the output switches of the inverter, which further generate DC voltage pulses to the load.
These voltage pulses are stepwise voltage changes which are known to generate a capacitive charge and discharge currents. Cables that are connected from the output of an inverter to the load are somewhat capacitive against the earth and against the cables of other phases, and this capacitance is charged when the output voltage of the inverter changes abruptly.
When a load current is measured in the inverter, i.e. at the beginning of the cable, charge and discharge currents are seen as part of the measured total current. This causes errors in the measurement since only the current fed to the load is of importance to the control. Effects of capacitive stray currents have the most dominant role when cables are long and the load is low-powered. In such a situation, the capacitance is large due to the long cables and the nominal current of the load being low. This leads to a situation where the magnitude of the stray currents is at its highest when compared to the load current.
When each phase current is measured separately, the above problem are avoided by timing the measurement such that the effects of the stray current are decreased. This can be carried out for example by measuring the current at a specific instant which is long after the previous change of the inverter output state.
It is known in the art that the output current can be measured in an intermediate voltage circuit. All current outputted from an inverter is flown through the intermediate circuit. By measuring the current of the plus or minus voltage bus and by knowing the state of the output switches at the time of a current sample, it is possible to know to which phase the measured current flows. It is thus possible to reconstruct the phase currents one at a time from the output switch positions and current samples. In this measurement method, only one current transducer is needed for making DC current measurement cost effective and simple. Further, the use of DC current measurement requires little space since only one current transducer is needed. These advantages make the DC-current measurement an attractive choice.
In DC current measurement, current measurement is carried out when an active voltage vector, i.e. not a zero vector, is chosen since a DC bus carries no current when a zero vector is selected. Modulation schemes can be selected such that an active voltage vector enabling a current measurement is chosen at a predetermined time instant. This time instant in relation to a modulation period is usually at the beginning or in the middle of each period.
The most problematic situations for DC current measurement are those with a low output frequency, where a generated output voltage is low and a modulation pattern only comprises short periods where an active voltage vector is in use. This automatically means that possible time instants for the measurement of current are timely in close proximity with voltage changes, and the measured currents include, in addition to a load current, disturbing amounts of stray currents. This situation is described in FIG. 1, where the upper three plots represent output voltages of phases A, B and C, while the lower three plots represent a total current measurable in the DC-bus, capacitive stray current and load current. A possible current measurement period is marked with a two-ended arrow. This period is the time in the modulation period during which the voltage vector is active and not a zero vector. As seen in FIG. 1, oscillations caused by the charge and discharge currents do not decay during the possible current measurement period. Thus, the problem relating to DC current measurement is the inability to measure the load current only.